Of Young Stars and Ancient Planets

by Paul Gilster on December 1, 2007

Since we’ve just been looking at young stars — protostars, at that — the news from Ann Arbor seems timely. Astronomers at the University of Michigan are announcing systems around UX Tau A and Lk Ca 15, young stars each, located about 450 light years away in the Taurus star formation region. What they’re actually observing at infrared wavelengths are gaps in the protoplanetary disks around these stars, the assumed result of planets sweeping the area clear of debris.

Unlike the infant star-in-the-making we looked at yesterday, UX Tau A and Lk Ca 15 are old enough — about a million years each — for planetary formation. Both are still pre-main sequence, deriving their energy from gravitational contraction instead of hydrogen-to-helium burning. To reach any conclusion about what’s happening around them, the Michigan team has to rule out photoevaporation, which is what happens when the dust and gas of a protoplanetary cloud heats up, evaporates and begins to dissipate. Catherine Espaillat rules out photoevaporation as the primary cause for what these Spitzer results have shown:

“Previously, astronomers were seeing holes at the centers of protoplanetary disks and one of the theories was that the star could be photoevaporating that material. We found that in some stars, including these two, instead of a hole, there’s a gap… It’s more like a lane has been cleared within the disk. That is not consistent with photoevaporation. The existence of planets is the most probable theory that can explain this structure.”

The paper is “On the Diversity of the Taurus Transitional Disks: UX Tau A & Lk Ca 15,” Astrophysical Journal Letters December 1, 2007 (full reference when it becomes available). An abstract is here.

Related: I sometimes ponder, as we look at systems in their infancy, whether some ancient civilization once looked on as our own Solar System formed. Some theorists, Charles Lineweaver prominent among them, have looked at how far back Earth-like planets might have formed, indicating the possibility of planets like ours fully nine billion years old, with peak planet formation about 1.8 billion years before our Sun came on the scene. So the possibility is there that we were once under infrared study from a distant world before Sol’s planets had even formed.

Or maybe not. A new paper from researchers at Southern Federal University (Russia) takes a look at star formation in terms of the appearance of metals, the latter being connected with the formation of planets. I haven’t worked through the entire paper yet (and there seem to be some problems in translation, as seen in the following excerpt), but as best I can make out, this statement from the paper’s abstract implies a different take on planetary development than Lineweaver:

Approximately 5 billion years ago average metallicity began to systematically increase, and its dispersion and the average relative magnesium abundance – to decrease. These properties may be explained by an increase in star formation rate with the simultaneous intensification of the processes of mixing the interstellar medium in the thin disk, provoke possible by interaction the Galaxy with the completely massive by satellite galaxy.

What I take from this is that the Russian team believes that stars like ours may not have been able to form in our area of the galaxy until about the time the Sun first appeared. Adam Crowl, who passed the reference along to me, points out that this would offer a take on the Fermi Paradox: They’re not here because we’re the first, or at least we’re early on the scene in a galactic neighborhood that’s relatively young in terms of living planets. It will be a while before we have a firmer grasp on just how long ago terrestrial planets might have formed in nearby space.

The Russian Team seems to be proposing a pretty anthropic argument here, I’m not too sure about buying into that.

I’m going with the odds that Earth-type planetary formation happened 9 billion years ago and the 1.8 billion years before our Sun ignited. Given what we’ve discovered about planetary formation from the exoplanets we’ve discovered so far, I’m going with the odds that at least one civilization either reached a technological singularity or launched star probes.

And even if this Russian team is right, it still reminds me of the book and basic idea of “Coming of age in the galaxy”, namely that there is presently a large population of similar type and age stars reaching an approximately similar stage of evolution at approx. the same time.

Still promising.

BTW, there are indications that the Andromeda galaxy is somewhat further in its evolution with a lot more main sequence (including sunlike) stars in a ‘mature’ stage and less remaining supernova and star formation activity (see Oxford Astronomy Encyclopedia and ‘W. Liller, B. Mayer (July 1987). “The Rate of Nova Production in the Galaxy”. Publications Astronomical Society of the Pacific 99′.

…Alan Bellows points out, rather forcefully, that pretty much all our radio transmissions – bar RADAR beams – would be indistinguishable from noise before even leaving the solar system. I found that rather surprising since I had accepted most of our transmissions would be still detectable from the nearby stars. After all we still pick up a tiny 24 watt signal from Voyager 1 & 2 from tens of billions of kilometres.

So ET might not know we’re here after all – all They might “see” would be the occasional “Wow!” signal from a RADAR sweep. I emailed Bellows for some details, so we’ll see how much he can elucidate his figures for us.

Adam: interesting implications for the METI debate there. An often-heard argument is that the aliens would have detected us already through our radio broadcasts, so we don’t have to worry about METI efforts revealing our civilisation. If the majority of our transmissions are undetectable before they leave the solar system.

I guess the difference here is between multidirectional and beamed signals – the typical leakage from our transmissions are not beamed, so they attenuate quickly, whereas beamed transmissions (like radar or the communications to the Voyager probes) fall off less rapidly with distance.

Seth Shostak has a paper out (unfortunately I don’t have the reference at hand) arguing that extraneous signals like our old TV shows would quickly dissipate, making them highly unlikely to be received in nearby solar systems. He discusses this also in a recent Space.com story:

With enough time (patience) and some idea of the characteristics of the signal to be recovered, it is possible to dig deep into the noise to retrieve a statistically-significant signal. A nice strong coherent long-duration carrier works well, and perhaps with a modulation rate well below 1 Hz.

Regarding us detecting Voyager vs. ET detecting us, there is an asymmetry at work. When we look at Voyager, or really any of our spacecraft, it’s set against a dark background. When ET looks our way, the background is the Sun’s emissions since it’s in a line with us. That lowers the signal-to-noise for pretty much any frequency they’d care to investigate. Same thing if we try to detect a planet-bound ET transmission, whether accidental or intentional. Doesn’t make it impossible, but more challenging.

Abstract: There is a close interrelation between Searching for Extraterrestrial Intelligence (SETI) and Messaging to Extraterrestrial Intelligence (METI). For example, the answers to the questions “Where to search” and “Where to send” are equivalent, in that both require an identical selection from the same target star lists. Similar considerations lead to a strategy of time synchronization between sending and searching. Both SETI and METI use large reflectors. The concept of “magic frequencies” may be applicable to both SETI and METI. Efforts to understand an alien civilization’s Interstellar Messages (IMs), and efforts to compose our own IMs so they will be easily understood by unfamiliar Extraterrestrials, are mutually complementary.

Furthermore, the METI-question: “How can we benefit from sending IMs, if a response may come only thousands of years later?” begs an equivalent SETI-question: “How can we benefit from searching, if it is impossible now to perceive the motivations and feelings of those who may have sent messages in the distant past?” A joint consideration of the theoretical and the practical aspects of both sending and searching for IMs, in the framework of a unified, disciplined scientific approach, can be quite fruitful. We seek to resolve the cultural disconnect between those who advocate sending interstellar messages, and others who anathematize those who would transmit.

I’d like to put forward another possibility as to why we’re not picking up any ‘chatter’

Perhaps the length of time a civilisation uses radio as a major means of communication is typically very short. Perhaps shortly after the discovery of radio (150-200 years?), a civilisation typically starts to use quantum effects to communicate information, which our radio disks can’t pick up. So not only are we trying to sort through a very large and naturally noisy universe we’re also looking for a radio signal from a very small time ‘window’.

It may be our radio dishes are the subject of many a joke on the galactic quantum internet (‘LOL look at what the humans are doing!’), but we wouldn’t know anything about it – indeed the skies could be filled with chatter but we’re still looking for ‘pigeon mail’, which just makes the joke even funnier.

@abc:
even the entire lifespan of a technologically advanced civilization may offer only a very narrow and rare window of opportunity. We just don’t know this (average) lifespan, the topic of many discussions.
Is advancement of a civilization to a higher (Kardashev) level almost inevitable, becoming virtually indestructable at and beyond interplanetary, or rather interstellar level.
Or does a tech. civ. usually destroy itself as a kind of evolutionary anomaly?

Question: when you say ‘quantum effects to communicate information’, do you mean something like quantum coupling (quantum entanglement), which indeed does not seem to be bound by any speed limits?

BTW: I do have some objections against the traditional Kardashev levels: I find them rather to crude, jumping from earth (level 1) to entire solar system (2) to entire galaxy (3)!
I would say that there is a much more important leap from solar system to interstellar, even if it means only one other star. This is probably *the* most crucial leap for any civilization in order to perpetuate itself.
Furthermore, I think that “harnessing the power of …” as a criteria (as Kardashev uses) is not as important as “having established a presence at” (earth, other planets within own system, other planetary systems, etc.). E.g. it may very well appear to be possible for us to reach other planetary systems and establish a permanent colony there (a landmark event in human history and survival) long before we harness most of the power of our solar system. We may never even have the need to do the latter.

Abstract: We investigate the behaviour of dust in protoplanetary disks under the action of gas drag using our 3D, two-fluid (gas+dust) SPH code. We present the evolution of the dust spatial distribution in global simulations of planetless disks as well as of disks containing an already formed planet. The resulting dust structures vary strongly with particle size and planetary gaps are much sharper than in the gas phase, making them easier to detect with ALMA than anticipated. We also find that there is a range of masses where a planet can open a gap in the dust layer whereas it doesn’t in the gas disk. Our dust distributions are fed to the radiative transfer code MCFOST to compute synthetic images, in order to derive constraints on the settling and growth of dust grains in observed disks.

Abstract: We present the first results of the treatment of grain growth in our 3D, two-fluid (gas+dust) SPH code describing protoplanetary disks. We implement a scheme able to reproduce the variation of grain sizes caused by a variety of physical processes and test it with the analytical expression of grain growth given by Stepinski & Valageas (1997) in simulations of a typical T Tauri disk around a one solar mass star. The results are in agreement with a turbulent growing process and validate the method. We are now able to simulate the grain growth process in a protoplanetary disk given by a more realistic physical description, currently under development. We discuss the implications of the combined effect of grain growth and dust vertical settling and radial migration on subsequent planetesimal formation.

Simulation of the Continuous Spectrum of Substars with Protoplanetary Discs

Authors: O.V. Zakhozhay, A.P. Vid’machenko, V.A. Zakhozhay

(Submitted on 10 Dec 2007)

Abstract: The continuous spectra of the substars with surrounding protoplanetary disks were calculated. The results reveal that protoplanetary disc average temperature decreases to 3 K during the period of 5 Myr for substars with masses 0.01M_{Sun} and during the period of 160 Myr for substars with masses 0.08M_{Sun}. Estimations of protoplanetary discs flux maximum depending on the substar mass at the age of 1 Myr are: 4.6 kJy (for 0.01M_{Sun}) and 3.4 MJy (for 0.08M_{Sun}). Maximum of protoplanetary disc radiation before it reaches the temperature of the cosmic microwave background changes within the ranges: from 0.07 mm to 0.58 mm (for substar mass 0.01M_{Sun}) and from 0.02 mm to 0.29 mm (for substar mass 0.08M_{Sun}).

Comments: Published in Proceedings of the 14th Young Scientists Conference on Astronomy and Space Physics, Kyiv, Ukraine, April 23-28, 2007

Abstract: We present observations of Taurus-Auriga Class I/II protostars obtained with the Spitzer InfraRed Spectrograph. Detailed spectral fits to the 6 and 15 micron features are made, using publicly-available laboratory data, to constrain the molecular composition, abundances, and levels of thermal processing along the lines of sight. We provide an inventory of the molecular environments observed, which have an average composition dominated by water ice with ~12% CO_2 (abundance relative to H_2O), greater than ~2-9% CH_3OH, less than ~14% NH_3, ~4% CH_4, ~2% H_2CO, ~0.6% HCOOH, and ~0.5% SO_2.

We find CO_2/H_2O ratios nearly equivalent to those observed in cold clouds and lines of sight toward the galactic center. The unidentified 6.8 micron profile shapes vary from source to source, and it is shown to be likely that even combinations of the most common candidates (NH_4+ and CH_3OH) are inadequate to explain the feature fully. We discuss correlations among SED spectral indices, abundance ratios, and thermally-processed ice fractions and their implications for CO_2 formation and evolution. Comparison of our spectral fits to cold molecular cloud sight-lines indicate abundant prestellar ice environments made even richer by the radiative effects of protostars. Our results add additional constraints and a finer level of detail to current full-scale models of protostellar and protoplanetary systems.

The UC Davis researchers estimate the timing of the formation of the carbonaceous chondrites at 4,568 million years ago, ranging from 910,000 years before that date to 1,170,000 years later.

by Staff Writers

Davis CA (SPX) Dec 20, 2007

UC Davis researchers have dated the earliest step in the formation of the solar system — when microscopic interstellar dust coalesced into mountain-sized chunks of rock — to 4,568 million years ago, within a range of about 2,080,000 years.

UC Davis postdoctoral researcher Frederic Moynier, Qing-zhu Yin, assistant professor of geology, and graduate student Benjamin Jacobsen established the dates by analyzing a particular type of meteorite, called a carbonaceous chondrite, which represents the oldest material left over from the formation of the solar system.

The physics and timing of this first stage of planet formation are not well understood, Yin said. So, putting time constraints on the process should help guide the physical models that could be used to explain it.

In the second stage, mountain-sized masses grew quickly into about 20 Mars-sized planets and, in the third and final stage, these small planets smashed into each other in a series of giant collisions that left the planets we know today. The dates of those stages are well established.

Carbonaceous chondrites are made up of globules of silica and grains of metals embedded in black, organic-rich matrix of interstellar dust. The matrix is relatively rich in the element manganese, and the globules are rich in chromium. Looking at a number of different meteorites collected on Earth, the researchers found a straight-line relationship between the ratio of the amount of manganese to that of chromium, the amount of matrix in the meteorites, and the amount of chromium-53.

These meteorites never became large enough to heat up from radioactive decay, so they have never been melted, Yin said. They are “cosmic sediments,” he said.

By measuring the amount of chromium-53, Yin said, they could work out how much of the radioactive isotope manganese-53 had initially been present, giving an indication of age. They then compared the amount of manganese-53 to slightly younger igneous (molten) meteorites of known age, called angrites.

The UC Davis researchers estimate the timing of the formation of the carbonaceous chondrites at 4,568 million years ago, ranging from 910,000 years before that date to 1,170,000 years later.

“We’ve captured a moment in history when this material got packed together,” Yin said.

Abstract: Giant planets embedded in circumstellar discs are expected to open gaps in these discs. We examine the vertical structure of the gap edges. We find that the planet excites spiral arms with significant (Mach number of a half) vertical motion of the gas, and discuss the implications of these motions. In particular, the spiral arms will induce strong vertical stirring of the dust, making the edge appeared `puffed up’ relative to the bulk of the disc. Infra-red observations (sensitive to dust) would be dominated by the light from the thick inner edge of the disc. Sub-millimetre observations (sensitive to gas velocities) would appear to be hot in `turbulent’ motions (actually the ordered motion caused by the passage of the spiral arms), but cold in chemistry. Resolved sub-millimetre maps of circumstellar discs might even be able to detect the spiral arms directly.

German astronomers reported this week that they have
discovered the youngest extrasolar planet found to date, a
world that is still embedded within the protoplanetary disk
around its star. Astronomers detected the presence of the
exoplanet surrounding the star TW Hydrae indirectly, using
the radial velocity technique used to find most of the over
250 known exoplanets. The star itself is estimated to be
only 8-10 million years old, and is still surrounded by a
protoplanetary disk of dust and gas. The planet, about 10
times the mass of Jupiter, orbits in an inner gap in the
disk, completing one orbit in just over 3.5 days. The
discovery puts new constraints on how long it takes for
planets to form around new stars.

Using ESO’s Very Large Telescope Interferometer,
astronomers have probed the inner parts of the disc of
material surrounding a young stellar object, witnessing
how it gains its mass before becoming an adult.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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